1. Field of the Invention
This invention relates to an improvement in air-operated combination diaphragm spring brake actuators of the type used with air brake systems on vehicles such as trucks. In one of its aspects, the invention relates to a spring brake actuator having a tamper-resistant spring chamber.
2. State of the Prior Art
Spring-applying brake actuators are in common use with air brake systems used on trucks, buses, and towed vehicles. Such actuators are normally provided with a service chamber for normally applying and releasing the brakes in response to delivery and exhaust of compressed air, and a spring chamber disposed in tandem with the service chamber for providing parking or emergency brake functions. A spring brake actuator uses spring force to operate a service brake actuator and apply brakes when pressurized air in the spring chamber is reduced below some predetermined level. Air pressure may be reduced in the spring chamber to apply the brakes under the control of the operator or automatically as a result of failure of the air system. The service chamber and spring chamber are separated by an adapter or flange casing which forms a wall between the two chambers.
In a typical spring brake, a barrel-shaped power spring is used to store energy and to exert the large force required for braking in the event of air pressure failure. Air pressure acting on a diaphragm or a piston is employed to compress the spring and maintain it in its brake release position. When the air is exhausted, the spring acts on a pressure plate which in turn acts against the diaphragm, typically an elastomeric diaphragm or a piston, and through an actuating rod exerts the spring force on the service push rod to apply the brakes in the event of a failure of the system air pressure.
The spring brake actuator operates within the spring chamber, which is typically formed by clamping an elastomeric diaphragm between a head (sometimes also known as a spring housing or spring chamber) and the adapter. The power spring is typically compressed within the spring chamber between the head and the diaphragm. The spring has a high spring constant and typically weighs 3 pounds or more being compressed to a linear length of less than 3 inches from an original uncompressed length in an extended condition of from 9 to 12 inches. With a high spring constant, the spring has a substantial amount of potential energy, exerting a force on the head of from 2,000 to 3,000 pounds.
Various approaches have been heretofore proposed for sealing the brake actuator head to the adapter. Typically, a clamp band is used to clamp mating flanges on the head and adapter to seal the spring chamber. To prevent disassembly of the two parts, the clamp band may be formed from a continuous ring, and deformed over the flanges. Alternatively, one flange may be deformed over the opposing flange.
Several designs have employed a ring having an interference fit with both the adapter and head to hold the two together. For instance, U.S. Pat. No. 3,107,583 employs a split snap ring in a piston-operated brake actuator which fits into a groove on the inward face of the adapter to hold the head in place against the force of the power spring. The snap ring is exposed to the elements and may be removed to disassemble the head from the adapter. In European Patent Application Pub. No. 313 217 A1, the snap ring is replaced by a locating ring comprising a hard core such as a coil spring encased by a watertight elastomeric coating. The locating ring has an interference fit between an annular groove on the inward face of the adapter and a lip on the head. The locating ring may be removed by moving the head inwardly of the adapter so that a smaller diameter portion of the head is opposite the annular groove. The locating ring may then be compressed out of the groove and removed. Neither design provides an effective tamper-resistant closure which significantly inhibits manual removal of the head.